Home Innovation Insights

Builders need to strike the right balance between high energy efficiency and durability.

Vladimir Kochkin
March 20, 2014

Green Homes Are Expected to be Durable (Now Let’s Make Sure)

High-performance homes appeal to consumers because of their touted benefits: a healthier indoor environment, lower operating costs, and increased durability. Certainly, the benefits are compelling. But do we know if high-performance homes can live up to these elevated expectations?

Unmet expectations are one thing. Performance failures are a whole other issue. New design and construction techniques can cause unintended impacts. A tight building envelope is good because it keeps conditioned air inside. But a tight building envelope can also lead to increased humidity and reduced drying of water that finds its way into an assembly. Humidity in moderation is good – but excess moisture is durability’s single biggest threat.

Builders are less able to rely on tried-and-true construction methods because of successively higher energy codes. Both the 2012 IECC and the 2015 IECC require light-frame wood builders to increase wall insulation and to reduce wall air leakage. The long-term moisture performance of these new wall systems is not well understood with regard to vapor drive, condensation risk, and drying capability. A wide range of new materials and systems is now available in the market to help builders comply with the higher energy codes. The new wall systems are more complex and interactions of these newer materials with each other and with conventional construction techniques are not obvious. Climate zone and HVAC equipment will also influences the home’s performance.

Home Innovation Research Labs is leading nationwide research to help architects, building scientists, and builders better understand the performance of energy-efficient wall systems. Our research involves monitoring walls in controlled test huts and in 22 homes in various climate zones. Our study includes different energy-efficient wall systems to better understand the impact, if any, of various types of exterior foam insulation, cavity insulation, interior vapor retarders, and framing sizes. Our methodology calls us to measure the moisture content of the sheathing and framing, cavity relative humidity and temperature, and interior and exterior relative humidity and temperature.

Due to space limitations, the following has been condensed to a summary of key points. For a fully detailed account of our methodology, conclusions, and recommendations, please download a copy of the full report.

What Have We Learned? Climate Zone 4 Test Huts Preliminary Lessons

The combination of three variables – (1) high interior relative humidity, (2) high permeance of the interior vapor retarder, and (3) air leakage path into the cavity – can cause exterior OSB sheathing to have high moisture content in Climate Zone 4. Further study is needed to uncouple the impact of air leakage from the effect of vapor diffusion, and to complete the assessment of the appropriateness of the ASHRAE 160 Simplified Relative Humidity (RH) protocol for evaluating wall performance. That additional research is currently underway.

Our findings suggest that the ASHRAE 160 Simplified RH protocol leads to wall moisture levels higher than those observed in the field. The effectiveness of a standard primer and paint to serve as Class III vapor retarder needs reevaluation as the drywall’s measured permeance (primed and 2 coats of paint) was above 30 perm.

Exterior foam does not appear to significantly impact OSB drying rates in the spring. 1-inch XPS exterior rigid sheathing (R5) has a marginal impact on the OSB sheathing’s moisture content in walls with vinyl siding and interior Kraft vapor retarder.

Walls with a Kraft paper interior vapor retarder are less sensitive to interior relative humidity conditions than walls without Kraft facing. On the other hand, interior relative humidity is a critical factor for wall design and performance when painted gypsum is the only interior vapor retarder. It appears that in addition to vapor retardant properties, Kraft paper helps to reduce the air flow into the wall cavity.

Computer simulations using WUFI software provide a good level of agreement with observed trends in moisture gain/drying.

Striking a Balance

The balance between high energy efficiency and durability is of great import to builders. As builders implement new and improved energy efficiency measures in their homes, they need assurance that the structure’s durability will not be compromised. Our research strategy is designed to help builders achieve superior energy efficiency, durability, and comfort for their specific climate regions in cost-effective and proven methodologies. Together, we aim to change the tide of innovation, ultimately creating better, more affordable homes for our country. If we can help you accomplish your goals for implementing innovation, please contact us.

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